The present invention pertains to packaging and passivation of microelectronic devices and more particularly to a technique for alternating current (AC) coupling of radio-frequency input and output signals to microelectronic devices contained in surface-mountable packages.
SAW devices are a class of microelectronic component which can perform a variety of radio frequency signal processing and conditioning functions. SAW devices may be formed on piezoelectric substrate materials or on more general materials if means are provided for electro-acoustic energy conversion, which can be comprised of a thin-film overlay of a piezoelectric material, as described in "SAW programmable matched filter signal processor," by F. S. Hickernell et al. (1980 IEEE Utrason. Symp. Proc., pp. 104-108). SAW devices operate by transforming an electrical signal into an acoustic surface wave signal through the piezoelectric effect. The surface acoustic wave signal is allowed to propagate along some portion of the substrate surface. The resulting surface acoustic wave propagation properties of the substrate provide the desired SAW device function.
Accordingly, it is important in employing these devices to avoid unwanted modification of the sound propagation properties of the surface of the substrate material. These properties can be substantially degraded through contact with foreign materials, or by damage to the substrate surface. Intimate contact with viscous solids or fluids, such as liquids and to a lesser extent gasses, results in substantial attenuation of surface acoustic waves, thereby greatly reducing the amplitude of an acoustic wave propagating on the surface of the substrate material.
SAW device substrates typically exhibit a variation of the time delay of a SAW signal in propagating from one point on the substrate to another point on the substrate versus temperature. This quantity, known as the temperature coefficient of delay (TCD) can be measured and is a property of the orientation and the composition of the SAW substrate employed. The TCD is influenced by the properties of any layers of material disposed over the SAW substrate surface, as is discussed in "SAW properties of SiO.sub.2 128.degree. Y-X LiNbO.sub.3 structure fabricated by magnetron sputtering technique," K. Yamanouchi and S. Hayama (IEEE Trans. Son. Ultrason . . . , Vol. SU-31, No. 1, Jan. 1984, pp. 51-57).
The non-zero TCD of typical SAW substrate materials results in changes in the electrical delay from the input to the output of a SAW device versus temperature. This occurs due to both anisotropic expansion of the substrate with temperature and changes in the elastic constants of the substrate with temperature. The thermal expansion properties of the SAW substrate often must be closely matched to those of the material on which the SAW device is mounted, both to avoid mechanical deformation and breakage of the SAW device with temperature and to avoid complex changes in the electrical performance which may occur due to the stresses produced in the SAW device by changes in temperature. Similarly, the TCD of the substrate may limit the applications for which a particular SAW device is suited.
SAW devices typically include means for absorption of acoustic energy in regions where it is desired to prevent acoustic wave propagation. These are included to avoid distortion of the device response by acoustic waves which have been reflected by the edges of the SAW device substrate, or by other features of the SAW device. Such reflected acoustic waves can result in unwanted acoustic energy echoes. These acoustic absorbers often are viscous materials such as silicone-based room temperature vulcanizing rubbers which are applied to the device surface by hand labor.
One approach to preventing unwanted contact of fluids and other undesirable material to SAW device substrate surfaces is to place the SAW device in a hermetically sealed package. Electrical connections to the devices can then be made by means of wires bonded to the SAW device and to electrically conductive pins which effect electrical contact to external electronic circuits. This electrical contact is often realized through insertion of the portions of the pins which are external to the package into suitably prepared holes drilled through circuit boards and by then soldering the pins to the boards.
Such practices are not well suited to automated manufacturing of electronic products. This is due to bending of the pins in shipment and also during insertion of the pins into the circuit board. The bond wires employed to electrically connect the SAW device and the pins require an additional manufacturing operation and are susceptible to breakage during the service life of the finished component. Also, the bond wires and the SAW device bus bars or bonding pads often are required to be made of dissimilar metals. Contact between dissimilar metals is a well-known cause of interconnection failure in microelectronic devices due to the formation of inter-metallic compounds.
These manufacturing methods also provide SAW device packages which have much greater volume than the SAW substrate material employed. The additional volume is required in order to accommodate the volume of the acoustic absorber material on the SAW substrate as well as the bond wires and pins.
Thus, it is desirable to have a SAW packaging method which provides a hermetically sealed environment for the SAW die, which allows external electrical connections to be made to the SAW die, which further provides the advantages of occupying as little volume as possible, enabling assembly of the packaged SAW components en masse in order to minimize manufacturing costs, and which allows the completed component to be used by automated circuit assembly machinery.